Dynamically positioned ships and vessels (such as a sea-based offshore platform, an oil platform or a drilling platform or a drillings rig) may use their propellers or thrusters to stay in position where stationary operation is required and vessels of this type may include drilling and/or production rigs for conveying oil and/or gas, e.g., from a drilling hole at the sea ground. For using electric propulsion (propellers or thrusters) these vessels may depend on a reliable power generation and distribution to supply electric propulsion machinery with electric energy, in order to maintain a desired position at the sea and in order to thus achieve a safe marine operation. For example, such kind of vessels Dynamic positioning (DP) may be used in water depths and areas where jack-up rigs and anchoring systems are not feasible. A conventional power generation system or power distribution system may be designed such that several isolated power distribution groups (typically 2-8) are operated so that if one group fails the remaining groups may be sufficient to hold the vessel in position. For example, in a conventional system redundant power generation (such as a large number of generators) may be required within several groups or all groups of the conventional power generation system.
For example, on some vessels, redundant converters and DC-links may be used in a conventional system to obtain a fault tolerant power system by connecting the energy flow between the isolated power generation and distribution islands using converters and DC-links. This may eliminate or at least reduce the problem of running excessive number of generators but may produce an expensive array of converters. Thereby, the additional converters may add significantly to the cost of the power generation system or power distribution system. This power distribution system may also be more maintenance-intensive and may contain additional sources of failures. Using a vessel management system, such as an automation system as part of the protection scheme, has been evaluated for some projects, but has been abandoned partly due to issues with response time. It may not be desirable to depend on additional systems for electric fault protection. This may also be unacceptable to classification societies and many customers.
Document EP 1 940 002 A2 discloses a relay device and a corresponding method, wherein a first value to the direction of a fault current flowing into the partial differential zone is assigned, a second value to the direction of the fault current flowing out of the partial differential zone is assigned, the values assigned to the fault currents are compared and it is determined, if the fault currents are flowing into the partial differential zone. Thereby, each node interfaces with the central processor 304 to provide node data for processing, decision making and the like.
Document U.S. Pat. No. 3,553,968 discloses a stabilized offshore platform located in a relatively deep body of water and supported by rigidly fixed upright legs.
Document EP 1 335 470 A2 discloses a directional comparison distance relay system for protecting a power transmission line including a directional distance relay element for deciding whether a fault is in forward or reverse directions.
There may be a need for a power distribution system for a dynamically positioned vessel and for a method for power distribution which is improved regarding fault handling and which at the same time does not require a large number of generators or converters. Further, there may be a need for a power distribution system and for a method for power distribution, wherein a single failure or fault within the power distribution system may not result in the vessel drifting away from its position.
Differential protection is often used in networks to isolate a faulty bus. On vessels involved in oil production and drilling, transient conditions may cause erratic trips by differential protection systems. These transient conditions are caused by starting of large motors and loads on a smaller, isolated electric power network causing larger fluctuations in frequency, voltages and harmonic content than what is normally observed for land-based electrical systems.